The present invention relates in general to boiler construction methods and arrangements and, in particular, to a new and useful method and apparatus for protecting tubes that are connected to a mud drum.
Industrial power boilers are commonly equipped with a boiler bank having an upper steam drum and a lower mud drum connected to the steam drum by a series of interconnecting steam generating tubes. For additional details of such boiler constructions, the reader is referred to Chapter 1, page 1-8 of Steam/its generation and use, 40th edition, Stultz and Kitto, Eds, Copyright(copyright) 1992, The Babcock and Wilcox Company.
These boiler or steam generating banks (as the structures are also called) suffer from corrosion at the tube-mud drum interface due to OD deposits that occur in this location. There are no methods or arrangements known to the inventor for preventing this corrosion and the only remedy is to replace the corroded tubes.
U.S. Pat. No. 5,236,524 to Rawers et al. discloses a method for improving the corrosion resistance of a zirconium-based material by laser cladding. A laser beam is scanned across the entire surface of the material to cause surface melting of the material. A rapid self-quenching is provided by the underlying substrate. Homogeneous material formed during solidification of the molten pool improves the corrosion resistance of the material.
U.S. Pat. No. 4,294,631 to Anthony et al. discloses a method for improving the corrosion resistance of a body of zirconium alloy to high pressure and high temperature steam. A scanning laser beam heats a surface region substantially equally, without melting, to a temperature range sufficient to form a barrier layer of corrosion resistant beta-quenched zirconium alloy at the treated surface.
U.S. Pat. No. 6,060,686 to Jones discloses a laser welding or cladding method. The main purpose of the laser cladding process is to overlay the surface of a substrate with another material having a different chemistry by melting a thin or thick interfacial layer to produce a metallurgical bond with minimum dilution of the clad layer. Laser surface cladding is a process in which powder or wire of different compositions is delivered into the laser-generated melt pool. The powder or wire is also melted by the laser beam, thereby forming a layer of clad alloy having a desired thickness and a chemistry that is different from that of the substrate. Among the advantages of this technique are the ability to produce novel alloys, minimized clad dilution, reduced alloy material loss, reduced machining, and reduced distortion. Conventional laser welding occurs in the ambient atmosphere, typically using a suitable inert cover gas.
U.S. Pat. No. 6,046,426 to Jeantette et al. disclosed a method and system for producing complex-shape objects by laser cladding of materials.
U.S. Pat. No. 5,569,396 to Topolski discloses a method for making alloying additions to a weld overlay weld pool. The weld overlay process is well-established and has been in commercial use for many years. Several common welding processes used in weld overlaying include: submerged-arc, conventional or pulsed gas metal arc welding (GMAW), cold or hot wire gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW), flux-core arc welding (FCAW), plasma transferred arc (PTA), laser welding, and electron beam welding. Typical applications include the cladding of tubes, pipes, flanges, and fittings with a corrosion-resistant layer. Additionally, the sealing and wear areas of valves and pumps may be clad for wear resistance. In addition to conventional arc welding processes, this reference teaches that laser or electron beam welding can be used to form a weld pool. The weld pool region is typically protected from oxidation by either using a gaseous shield or vacuum. In the process, the filler metal may also conduct the current to establish and maintain the welding arc (consumable electrode) or it may be separately fed (cold wire) into the arc or weld pool. The form of the filler metal can either be a wire, powder, or strip. The composition of the weld pool is a function of the composition of the filler metal and dilution by the metal component. The resultant corrosion or wear-resistant weld overlay clad layer is generally a function of the weld pool""s composition.
One aspect of the present invention is drawn to a method for protecting the ends of steam generating tubes from corrosion at the tube-mud drum interface, a location that is particularly susceptible to corrosion, and the tubes produced by that method. Thus, one aspect of the present invention is drawn to a method of corrosion protecting a tube having an end portion extending into a tube receiving hole of a mud drum of a boiler, comprising: laser cladding a corrosion resistant cladding on an outside diameter of the tube along a length of the end portion of the tube.
Another aspect of the present invention is drawn to a tube having a corrosion resistant end portion for extending into a tube receiving hole of a mud drum of a boiler, comprising: a corrosion resistant laser cladding region on an outside diameter of the tube along a length of the end portion of the tube.
The tapered laser cladding region is provided on the outside diameter (OD) of the tube, prior to installation in the tube receiving hole in the mud drum, and in the area immediately above a hole in the mud drum which receives the tube. The tapered laser cladding region also extends partly into the hole, but does not extend into the rolled area of the tube.
According to the present invention, the alloy or alloy combination of either the tubes or the mud drum is not critical. The required thickness and composition of the cladding itself will depend on the corrosive environment to which the boiler mud drum and steam generating tubes are exposed and the degree to which such corrosion must be avoided. Examples of alloys for the tubes and boiler can be found in the above-identified publication Steam/its generation and use. Any corrosion resistant coating can be used for the tapered corrosion protection, but generally a high chromium content alloy which is either ferritic or nickel based is appropriate. The coating thickness may be on the order of 0.07 inches or less, tapering from a maximum thickness of about 0.10 inch to about 0.05 inch, gradually tapering to a thickness of 0.0 inch at the end of the tapered cladding portion which is within the tube receiving hole in the mud drum.
The thickness of the cladding must be controlled, however, to avoid interference between the clad tube and the drum hole for easy fabrication and attachment of the tubes to the drum. Thick coatings must not protrude into the mud drum, but must taper to allow the tube to be easily inserted into the hole to a depth sufficient for attaching the tube to the mud drum. As such, laser cladding is particularly useful for the present invention in that it is uniquely adapted to place the corrosion resistant cladding onto the tube in a tapered fashion.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific benefits attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.